Re: Calculating skin effect using FEMM?

[k.gregory@xxxxxxxxxxx]

Dave,

Long time no e-mail but I have been using FEMM a lot lately and 
recommending it to everyone!

Anyway, I'm a little confused about a bit of one of your replies, 
maybe I'm being a bit stupid, but you said

The multiple bars per phase illustrates an interesting feature of 
the "circuit" properties--if you label multiple blocks with the same 
circuit, it assumes that all blocks are driven in series, and that 
the _total_ current in _all_ blocks that are defined with a 
particular circuit property adds up to meet the constraint. If you 
examine the two bars composing each phase, you will find that each 
bar carries a slightly different current, but the total current adds 
up to be the desired phase current.

If the blocks are driven IN SERIES then shouldn't they carry the same 
current or if they add up to the right total current then aren't they 
in parallel?

Keith. 

--- In femm@xxxxxxxxxxx, dcm3c@xxxx wrote:
> In an attachment to message dated 1/5/01 7:37:10 AM EST, 
> dgorenc@xxxx writes:
> 
> > I am using "Finite Element Method Magnetics" programs for
> > calculation of eddy current losses in enclosure of three-phase
> > segregated and nonsegregated busbars. Thanks to you, until now
> > I have successfully calculated losses in enclosure for several
> > configuration of busbars.
> 
> > In FEMM Users Manual you said that for bulk coils ( in my case 
> > they carry currents Jsrc) conductivity of zero should be defined.
> > In that case I could calculate only a eddy current losses in
> > enclosure, and losses in conductors are zero.
> 
> What I mean by "bulk coils" are coils with many turns that are
> approximated as one block of constant current density. It
> isn't appropriate for the current to redistribute itself in an
> eddy current problem in a wound coil the way that it would in
> a solid bus bar, so you have to assign the conductivity to be
> zero so that the current distribution in the block is
> unchanged by eddy currents.
> 
> > Since the equation that FEMM actually solves for planar (2D)
> > problems is:
> > 
> > \nabla^2 A - j \omega \mu \sigma A = - \mu Jsrc (1)
> > 
> > and this is the same as:
> >
> > \nabla^2 A - j \omega \mu \sigma A = - \mu \sigma U (2)
> >
> > where ((U=Jsrc, and U is the voltage applied to a conductor in
> > which skin effect is to be calculated. (Equation (2) was taken
> > from "Teoretska elektrotehnika" by Z. Haznadar)
> > If I define a nonzero conductivity for conductors (bulk coils)
> > and define such voltage which will give the real value of
> > current through the conductors, I could actually calculate
> > resistive losses or skin effect factor in conductors as 
> > kskin=Pac/Pdc, where Pac are resistive losses for alternating
> > current, and Pdc losses for direct current.
> >
> 
> > To get the real current through the conductors (real amplitude
> > and phase, for example: for three phase current i1=I cos(t),
> > i2=Icos((t+120(), i3=Icos((t-120() or as phasor i1=I+j0,
> > i2=-0.5I+j0.866I, i3=-0.5I-j0.866I, where I is amplitude of
> > sinusoidal time varying current) I must do several iterations.
> > When I achieve real values of currents through the conductors
> > then I can calculate losses in conductors and losses in
> > enclosure. I made several calculations with and without taking
> > in consideration skin effect in conductors. In both cases the
> > eddy current losses in enclosure was the same. I also made
> > several calculations of skin effect in conductors (without
> > enclosure). For two parallel Al conductors 100x10 mm on
> > distance 30 mm through which alternating (in first
> > calculation) and direct (in second calculation) current of
> > 200A flows, I obtained kskin=1.22. For the same conductors in
> > some books I found kskin=1.19 and in others kskin=1.23. The
> > difference probably happened because electrical conductivity
> > of Al used in my calculation was little different than those
> > used in other Literature.
> >
> > Is it possible to calculate losses in conductors for
> > alternating current in this way using FEMM?
> 
> Your development is correct. In the 2.1a version of the
> program, this was the only way to approach eddy current
> problems in which a fixed current is desired in a conductive
> region. I kept running into eddy current problems where
> I needed to place constraints on the total current flowing
> in a set of conductive regions in an eddy current problem, so
> I fixed in this in the 3.0 Beta version. Unfortunately, I
> haven't gotten around to documenting it in the manual yet.
> 
> Anyhow, the "Circuit" property in the 3.0 version is just 
> what you need. The name "circuit" is sort of a misnomer;
> this property really just allows one to apply constraints on
> the total current carried by a block or group of blocks. What
> the circuit properties actually do is apply an additional
> voltage to each block defined with a particular circuit
> property. Additional equations are then solved during the
> solution process so that the voltages are chosen to satisfy the
> current constraints--doing automatically what you have been
> doing manually. The circuit properties are located in the
> "Properties" part of the main menu. They are defined in a way
> that is conceptually similar to point, boundary, and material
> properties.
> 
> For your problem, you would want to make four separate
> "circuits": One for each of your three phases, and a fourth
> "circuit" for the aluminum enclosure. For the A, B, and C
> phases, you'd enter the total current as you have described
> above:
> 
> Phase A = I+j0;
> Phase B = -0.5I+j0.866I
> Phase C = -0.5I-j0.866I
> 
> The purpose of the circuit for the enclosure is to ensure that
> eddy currents induced in the enclosure are conserved within the
> enclosure. You'd just assign the total current for the
> enclosure to be:
> 
> Aluminum Enclosure = 0 + j0
> 
> Then, these circuit properties have to be applied to the
> appropriate areas of the solution. The circuit properties are
> applied to block labels in just the same way that material
> properties are applied to a block label. (i.e. select desired
> label with right mouse click; hit <space> to "open" the
> label's properties; choose desired circuit from the "In
> Circuit" drop list)
> 
> I have attached an example of your geometry that does just
> this, assuming that the current carried by each phase is 1000
> Amps RMS. I also assumed that the left-most pair of bars
> carries the Phase A current, the center pair the Phase B
> current, and the right-most the Phase C current. 
> 
> The multiple bars per phase illustrates an interesting feature
> of the "circuit" properties--if you label multiple blocks with
> the same circuit, it assumes that all blocks are driven in
> series, and that the _total_ current in _all_ blocks that are
> defined with a particular circuit property adds up to meet the
> constraint. If you examine the two bars composing each phase,
> you will find that each bar carries a slightly different
> current, but the total current adds up to be the desired phase
> current.
> 
> Another interesting postprocessing feature is that you can
> examine what voltage was required to enforce the current
> constraint. Just pick View | Circuit Props off of the main
> menu in femmview. A dialog will then pop up, displaying the
> current, voltage, and impedance for each circuit.
> 
> > In all of my calculations I used Dirichlet boundary condition.
> 
> That's fine, as long as the A=0 boundary is drawn "far enough"
> away from the region of interest so as not to disturb things.
> A lot of the time, I will use the "Asymptotic Boundary
> Condition" described in the "Open Boundary Problems" section
> of the appendix in the femm manual. This generally lets you
> get away with drawing the boundary fairly close to the parts
> that you are interested in without much loss of accuracy. I
> have used this technique for the outer boundary in the
> attached example.
> 
> Dave Meeker
> --
> http://members.aol.com/dcm3c